In the semiconductor industry, accurate measurement of the dimensions of a given feature is critical for process control and yield learning. Primarily, the dimension of a feature is measured using a critical dimension scanning electron micrograph (CDSEM) technique. In some limited applications, the dimension of a feature is determined using a scatterometer. In CDSEM, the probe is an electron beam, while light is used as the probe for a scatterometer.
Despite the different probes used, both techniques rely on reflections of the probes off the features under investigation, or electrons generated during the reflection interaction. The reflection can be modulated by the feature itself as well as many other undesired factors. The most common factors that may complicate the measurement are (i) process induced line edge roughness, (ii) process induced residue, and (iii) electrostatic charging induced by process and electron beam in CDSEM.
In addition, and with the current trend of shrinking lines and spaces, it is becoming a growing challenge to differentiate lines from spaces in CDSEM images. Without being able to reliably identify the lines or spacers, there is a risk of measuring from an undesired location, causing false process feedback and yield issues.
There are many activities in processing trying to address the above-mentioned problems. However, because CDSEM and scatterometer work off reflection interaction, it is not possible to eliminate the above challenges. Additional creativities are urgently needed to ensure that the measurement taken accurately represents the desired feature.
CDSEM beam spot size has traditionally be quantified offline during a calibration process by using gold-carbon samples at various limited CDSEM setup conditions. The data obtained from the calibration is then used as a reference (or look up) table. However, there are many, many variables in a CDSEM which may affect the beam spot size and it is nearly impossible to have a reference table for each and every condition.
In view of the above, there is a continued need for providing a new and improved means for accurately determining the feature size during semiconductor processing. A new and improved means for quantification of the beam spot size of a CDSEM at real time is also desirable.